The most significant progress, as far as an increase in performance of machines like turbines are concerned, can be realized by increasing the process temperature. However, such increase in process temperature can result in the fact that metallic components of the machine are stressed beyond the limits of their safe operating area such that these components will not endure an operation under these conditions for a long time without damage or at least alteration of their properties.
It is well known in the prior art to make use of coatings applied to such metallic components in order to protect them from such critical operating conditions. For example, ceramic heat shield coats are used to decrease the heat conductivity between process chamber and machine component, or metallic coats to increase the hot gas corrosion resistance of the surface of such metallic machine components. For many years, such coats are also applied by a thermal coating process; nowadays, they are state of the art.
Since in most cases one single coat is not sufficient to resist a complex stress attack,—particularly if the stress is extremely high—, preferably a coating system consisting of a plurality of different layers is applied; thereby, each layer has specific properties particularly suitable to withstand a specific stress. A typical example is to apply a stabilized zirconium dioxide layer, serving as a heat shield layer, onto a metallic layer that is resistant against hot gas corrosion, for example a MCrAlY-layer, whereby M represents a metal on the basis of cobalt, nickel or iron. Preferably, such a layer is applied directly onto the component to be protected.
Following the requirements regarding performance and life span, in the past further layers have been developed to be applied in addition to the two-layer-systems “Stabilized Zirconium Oxide/MCrAlY”. Since it can happen at high temperatures that a diffusion of important metal atoms occurs between the substrate and the MCrAlY-layer, the last named layer changes its properties in a negative sense until it cannot fulfill its function any longer. In order to prevent this side effect, an intermediate layer, located between the substrate and the MCrAlY-layer, has been developed, serving either as a diffusion barrier or as a donator of important metal atoms (designated in the following as “barrier layer”). A further intermediate layer is already used for the region between the MCrAlY-Layer and the barrier layer which reduces the oxidative attack to the MCrAlY-layer and improves the adherence to the barrier layer.